1. Field of the Invention
The present invention relates to a boring apparatus for boring perforations in rolled paper or a web sheet, such as a tipping paper sheet, and more particularly, to a boring apparatus capable of boring perforations of a uniform shape in a web sheet at high speed.
2. Description of the Related Art
In a conventional boring apparatus, a pulse laser beam is used to form a number of perforations with a uniform shape and accurate pattern in rolled paper or a web sheet, such as tipping paper sheet. Conventional methods for converting a laser beam into pulses include a pulse beam oscillation method (1), slit method (2), chopper method (3), and shutter method or beam scanning method (4).
In the pulse beam oscillation method (1), a carbon dioxide gas laser, which generates a pulsating laser beam, is usually employed as a laser beam source. The carbon dioxide gas laser is selected because it generates the pulsating laser beam with great power, and the generated laser beam has a wavelength of 10 .mu., the optimum wavelength for the absorption of water in a web sheet, e.g., tipping paper sheet. In the existing circumstances, it is believed that this gas laser, among other lasers developed so far, is best suited for boring perforations in paper. In the carbon dioxide gas laser, however, the first and last transitions of the laser beam require several hundreds of microseconds each, and a laser beam pulse train cannot be generated in a short enough period. In a boring apparatus using the carbon dioxide gas laser as its light source, therefore, the perforations cannot not be formed at high speed in an unlimited manner, that is, the boring speed cannot be increased. More specifically, the maximum speed for 1-mm pitch boring is only 2 mm/sec, so that the boring efficiency is not very high.
In the slit method (2), a slit member having a row of slits formed therein, along with a paper sheet to be perforated, is run on a boring plane in a manner such that a continuously emitted laser beam is condensed on the boring plane by means of a lens system, whereby the laser beam is intermittently condensed on the sheet through the slits. Thus, perforations are intermittently formed in the sheet corresponding to the row of slits. According to this slit method, in contrast with the pulse beam oscillation method, the laser beam is continuously oscillated. It is believed, therefore, that the perforations can be bored through the sheet at higher speed than in the case of the pulse beam oscillation method, without any restrictions on the boring speed associated with the first and last transitions of the laser beam, and the cost of boring operation can be reduced According to a specific example of the slit method, the sheet can be perforated at the boring speed of 6 mm/sec. This slit method, however, is subject to the following problems.
(a) Since the slits of the slit member, which are subject to variation in size, are transferred directly to the sheet, the size and pitch of the perforations vary depending on the working accuracy of the slits. In general, the slit method compares unfavorably with the pulse beam oscillation method in the variation in perforation size and pitch.
(b) The slits are situated close to the substantial focal point of the lens system, and the condensed laser beam is continually applied to the slit member. In some cases, therefore, the slit member may be damaged by the condensed laser beam. If the slit member is damaged, the individual slits are changed in size, so that the irregularity of the perforations is enlarged.
(c) In boring the perforations, the paper sheet is burnt by the laser energy. The resulting ashes of the sheet may possibly adhere to the slits, thereby hindering the passage of the condensed laser beam. As a result, the perforations are subject to greater variation in size.
In the chopper method (3), a continuously emitted laser beam is directed to a rotating chopper disk, and is chopped and divided into segment laser beams by means of the disk. As these segment beams are successively condensed on a paper sheet, perforations are intermittently formed in the sheet at regular intervals. Although the problems peculiar to the pulse beam oscillation method and the slit method can be solved, according to this chopper method, the chopper disk is expected to rotate at high speed, and may possibly be broken by centrifugal force produced by the rotation. More specifically, if the diameter of the chopper disk and the number chops are 30 cm and 20, respectively, only 20,000 perforations can be formed with every minute when the disk is rotated at the speed of 1,000 rpm. When the rotating speed of the disk is 60,000 rpm, on contrast with this, 1,200,000 perforations can be formed with every minute. If the chopper disk is rotated at the speed of 60,000 rpm, however, there is a high possibility of its being broken.
In the pulse beam oscillation method (1) described above, the laser beam is intermittently oscillated, so that the laser source itself cannot efficiently generate the laser beam. In the slit method (2) and the chopper method (3), the power of the emitted laser beam is not effectively utilized. More specifically, according to the slit and chopper methods, the laser beam is intercepted by the slit member or chopper disk for that region of the paper sheet which should not be perforated, so that the laser beam emitted from the laser is not used well.
In the shutter method (4), as is disclosed in Published Examined Japanese Patent Application Nos. 57-37437, 57-49318 and 59-318, a continuously emitted laser beam is directed to a plurality of rotating disks, which have a plurality of sets of reflective surfaces and openings each. As the laser beam is reflected by the reflective surfaces and transmitted through the openings, it is divided into segment pulses and condensed on a paper sheet to be perforated. According to this shutter method, a plurality of rows of perforations are formed simultaneously, so that the laser power can be effectively utilized for high-efficiency boring. In this case, however, it is necessary to use the rotating disks as many as the rows of perforations. For high-speed boring operation, moreover, these disks are expected to be manufactured with a very high working accuracy. If the working accuracy is low, the rotation of the rotating disks is ill balanced, so that the perforation accuracy may be lowered, and in the worst case, the apparatus itself may be destroyed. Since the plurality of sets of reflective surfaces and openings must be arranged on the rotating disks, furthermore, each disk usually should have a large diameter. Also for this reason, the working accuracy of the rotating disks may be lowered, thereby entailing lower perforation accuracy or breakdown of the apparatus.
The conventional boring apparatus, using any of the methods described above, is subject to problems such that high-speed boring operation is difficult, the perforations vary in size and are low in accuracy, and/or the laser power cannot be effectively utilized. When rotating disks with low working accuracy are used to perform high-speed boring operation, moreover, the apparatus itself may be damaged.